are basically colormetric devices in a tube.
They work for extreme situations, but whether they will warn you of low-level problems is in serious doubt, and like all other things at pressure, its the partial pressure that's the problem.
So if you have 50ppm of CO in the gas, that's bad. But at 4ATM its like 200 ppm, which is going to produce severe impairment and over enough time could even be fatal.
Grade E air permits 10 ppm of CO. But again, at 4 ATM that's 40 ppm, and that's above the limit where firefighters (35 ppm) will don their SCBA gear!
Will you have notable symptoms at 40ppm of PCO? I don't know. There are many who say "no", and that the lower limit is 100 ppm. Frankly, I don't know if I buy that or not.
I have one of the "ultra-sensitive" style units on my boat; while I have diesel engines and a diesel genset, and neither are commonly implicated in CO poisoning, we often anchor out and its the OTHER GUY next to us with a gas genset running that scares me - you could be sleeping when he pulls up and drops the hook and he could easily poison you without warning.
I also have one of the more "usual" units (less sensitive) in each cabin to detect immediately hazardous conditions.